2017 Volume 58 Issue 4 Pages 641-645
Subjected to hoop stress and land movement load, the orientation of cracks on the surface of pipeline may be axial direction, circumferential direction or the others. In this paper, an electromagnetic Helmholtz-coil probe is presented to detect the oriented cracks on the surface of pipeline. A structure of equal-spaced TMR sensor array of the Helmholtz-coil probe is applied to scan the full circumference of the pipeline simultaneously in a single pass. The simulation and experiment results indicate that using the combination effect of the electric current perturbation and the magnetic flux leakage, the oriented crack can be mapped clearly.
Pipelines provide the safest and most economical form of transportation of crude oil, natural gas, and other petrochemical commodities compared to truck, rail cars, and tankers1). Suffering from the degeneration of materials, for example crack, corrosion, it is important to carry out nondestructive testing (NDT) to maintain high reliability of pipeline transportation.
Surface cracks, especially the stress corrosion crack (SCC), are one of the most harmful degradations considering their effect on structural integrity2). Subjected to hoop stress and land movement load, the orientation of surface cracks may be axial direction, circumferential direction or the others. Ultrasonic testing is a widely used technique in pipeline detection. However, the need of couplant limits the detection in gas transportation pipeline3,4). As to magnetic flux leakage, the leaking magnetic flux is proportional to the opening of the crack, so it is not sensitive to axial tight SCC5–7). Magnetic particle inspection (MPI) is the most reliable NDT method in pipeline detection. While MPI is a very effective inspection technique, its field use can be costly when one considers the surface cleaning and operating time5). The eddy current (EC) could not make accurate size assessment of cracks8).
The aim of this paper is to propose an electromagnetic Helmholtz-coil probe for oriented cracks mapping and sizing on the surface of pipeline. This paper is organized as follows. In section 2, the model of the Helmholtz-coil probe is presented and the perturbation of the magnetic field above the crack is analyzed through the finite element software COMSOL. In section 3 the electromagnetic Helmholtz-coil probe is set up and the oriented crack in the range of 0° to 90° are mapped through experimental testing.
An electromagnetic Helmholtz-coil probe consists of two critical components was presented in this paper: a Helmholtz-coil excitation and a detecting sensor array. The Helmholtz-coil excited by alternating current (AC) was coaxial with the pipeline to induce a uniform current field along the circumference of the pipeline and a uniform magnetic field along the axis of the pipeline9). The tunnel magneto resistive (TMR) detecting sensor array was used to measure the magnetic field signals.
2.1 Finite element model of the electromagnetic helmholtz-coil probeThe finite element model of the electromagnetic Helmholtz-coil probe was built through the COMSOL software, as shown in Fig. 1. The “magnetic field physics” was selected and the “impedance boundary condition” was applied on the surface of the pipeline. The dimensions of the model are shown in Table 1 and the characteristic parameters are shown in Table 2. The electric current and magnetic flux density distribution on the surface of pipeline without crack are shown in Fig. 2. As shown in Fig. 2, the uniform electric current field and magnetic field are induced on the surface of pipeline10,11).
FEM model the electromagnetic Helmholtz-coil probe.
| Model | Diameter/mm | Length/mm | Width/mm | Depth/mm | Interval/mm |
|---|---|---|---|---|---|
| Pipeline(D/d) | 65/45 | 300 | |||
| Helmholtz-coil | 80 | 4 | 40 | ||
| Crack | 20 | 1 | 5 |
| Number of turns | Pipeline material | Conductivity/S/m | Permeability | Excitation current/A | Frequency/Hz |
|---|---|---|---|---|---|
| 500 | Carbon steel | 1.12e7 | 4000 | 1 | 1000 |
Electric current and magnetic flux density distribution on the surface of the pipeline.
In the simulation, the oriented cracks in the range of 0° to 90°, as shown in Fig. 3, were detected. The position of the probe was moved along the axial direction (z-axis) to simulate the probe scanning process along a pipeline during experimental studies. The response magnetic field of the probe was measured at a 1 mm lift-off. Two components of the magnetic field, axial direction (z-axis) Bz and radial direction (y-axis) By, were measured as the characteristic signals, as shown in Fig. 4.
Oriented cracks ranging from 0° to 90°.
Bz and By signals, (a) Bz, (b) By.
As shown in Fig. 4(a) and 4(b), with the increasing of the angle, the Bz signal changes from dip to peak gradually and the By signal remained a peak and dip. However, the order of the peak and dip in By signals varies with the increasing of crack orientation.
The electric current density around the 0° orientation crack on the surface of pipeline and magnetic flux density around the 90° orientation crack in the x-y plane of the model are shown in Fig. 5.
The electric current distribution and magnetic flux density around the crack, (a) electric current density around the 0° orientation crack on the surface of pipeline, (b) magnetic flux density around the 90° orientation crack in the x-y plane of the model.
It can be seen from the Fig. 5(a) that the electric current concentrating in the tips of the crack, which is expressed as arrow, when the crack of 0° orientation is detected is the electric current perturbation effect caused by discontinuous conductivity. As shown in Fig. 5(b), the magnetic flux leaks in the edge of the crack, which is expressed as line, when the crack of 90° orientation is detected is the magnetic flux leakage effect caused by discontinuous permeability. These phenomenon also explain the variations of Bz and By signal in Fig. 4 that the Bz and By signals of the crack of 0°, 15° orientation are caused by the electric current perturbation effect and the signals of the crack in the range of 30° to 90° orientation are caused by the magnetic flux leakage effect.
An implication of this is the possibility that the combination effect of the electric current perturbation and magnetic flux leakage around the crack may have the advantage of detecting the oriented crack on the surface of pipeline.
The structure of the Helmholtz probe is shown in Fig. 6. The excitation coils were wound on the polymer frame. Detecting sensors were equally-spaced installed in the polymer frame. Supports were used to fix the probe and keep the constant lift-off to different pipeline. The extension-type tape was used to fix the sensor array. A sensor array containing Tunnel Magneto Resistive (TMR) was employed to scan the full circumference of the pipeline. Considering the space of the Helmholtz-coil electromagnetic probe and actual manufacturing difficulty, a 24 equal-spaced sensor array was selected. An electromagnetic Helmholtz-coil detection system was built, as shown in Fig. 7. The excitation source produced an alternating current signal with the frequency of 1 kHz and the magnitude of 10 V. The turns of the coil were 1000 in total. The current was transferred to the excitation coil through the power amplifiers. The detecting sensor array picked up the magnetic field and translated it into electric signal. The signals were amplified and filtered in signal processing module. And then, the signals were converted into digital signal by an A/D convertor and sent to PC for signal processing. A detection software was developed to achieve defects recognition. The Bz and By signals were shown in the display screen of computer. The Helmholtz-coil probe was fixed in an axial scan table, as shown in Fig. 7.
Structure of Helmholtz-coil probe.
Experimental setup.
The pipeline made from carbon steel was detected to test the detectability of Helmholtz-coil probe to crack. The axial cracks, which are 1 mm width and 30 mm length, with the depth in the range of 2 to 10 mm were machined on the surface of pipeline by Electric Discharge Machining (EDM), as shown in Fig. 8.
(a) Schematic diagram of pipeline and (b) Actual cracks in pipe strings.
The pipe string was moved along the axial direction at a speed of 10 mm/s and the Bz and By signals were measured and shown in Fig. 9. It can be seen from the figures that all of the cracks on the surface of pipeline could be detected obviously. Furthermore, the length of cracks can be obtained accurately. The amplitudes of the signal are proportional to the depth of the cracks.
Bz and By signals.
Oriented cracks with the orientation in the range of 0° to 90° were machined on the surface of pipeline by EDM, as shown in Fig. 10, and the parameters of the cracks were 45 mm × 1 mm × 5 mm (length × width × depth). The pipeline was scanned through the system in Fig. 7 at a speed of 10 mm/s. The data of the sensor array were obtained and drawn in the MATLAB. The C scan results are shown in Fig. 11. It can be seen from the results that the orientation of the cracks can be recognized clearly. Moreover, with the increasing of the crack orientation, the Bz signal varies from dip to peak gradually. The Bz and By signals of the crack of 0°, 15° and 45° orientation are caused by the electric current perturbation effect and the signals of the crack of 60°, 75° and 90° orientation are caused by the magnetic flux leakage effect. Comparing with the results in simulation, we can see that the numerical and experimental results have the similar variation law and mechanism that using the combination effect of electric current perturbation and magnetic flux leakage, the arbitrary orientation crack can be detected.
Pipeline with the cracks in the range of 0° to 90° orientation.
C scan results of the oriented crack ranging from 0° to 90° with 15° interval, (a), (c), (e), (g), (i), (k), (m) are the Bz signals, (b), (d), (f), (h), (j), (l), (n) are the By signals.
In this paper, a Helmholtz-coil probe has been proposed to detect oriented cracks on the surface of the pipeline. A FEM model of this probe is put forward through the COMSOL. Based on the model, the relationship between the crack orientation and Bz and By signal is analysis. Finally, the structure of the Helmholtz-coil probe with a sensor array is designed and the experimental studies are carried out to test the detectability of oriented crack. The results of the simulations and experiments show that: using the combination effect of the current perturbation and magnetic flux leakage, the oriented crack on the surface of pipeline can be recognized clearly. The length of the cracks can be measured through the By signals.
Although, the detection and length measurement of the oriented cracks through the probe in this paper have been proved, the depth of the oriented crack can't be sized directly from the Bz signals. In the future work, we will focus on the depth sizing algorithm of oriented cracks through the Helmholtz-coil probe. Moreover, in the experiment, the artificial cracks were detected. In the next stage, some actual cracks or closed SCC will be tested.
Wei Li and Jiuhao Ge contributed equally to this work. This work was funded by the National Natural Science Foundation of China (No. 51574276 and No. 51675536), the Shandong Provincial Natural Science Foundation (No.ZR2015EM009), Special national key research and development plan (No.2016YFC0802300 and No.2016YFC0303800), and the Fundamental Research Funds for the Central Universities (No.16CX06017A, No. 15CX05024A and No. 14CX02198A).
